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Domestic wastewater treatment by down-flow hanging sponge (DHS) system: A sustainable and techno-economic approach
1, 2 , 1, 3 , 4 , * 1, 5
1  Egypt-Japan University of Science and Technology (E-JUST), Alexandria 21934, Egypt
2  Menoufia University, Menoufia 32511, Egypt
3  High Institute of Public Health, Alexandria University, Alexandria, Egypt
4  School of Environment and Society, Tokyo Institute of Technology, Tokyo, Japan
5  Alexandria University, Alexandria 21544, Egypt
Academic Editor: Bipro Dhar


The biological wastewater treatment processes utilize excess energy for aeration and pumping, and the generated sludge is associated with severe waste disposal issues. Hence, this study represents a cost-efficient and reliable method for treating wastewater without the requirement of external artificial aeration or additional sludge treatment steps. A down-flow hanging sponge (DHS) system occupied by sponge pieces, i.e., as a biofilm carrier, was established to treat domestic wastewater over 100 days of continuous feed. The DHS reactor was operated at a food-to-micro-organism (F/M) ratio of 0.27 kg COD/kg MLVSS/d, a sludge residence time (SRT) of 89 d, and a fixed hydraulic retention time (HRT) of 1.2 h. The results demonstrated that the removal efficiencies of chemical oxygen demand (COD), total suspended solids (TSS), ammonia (NH3), and total dissolved solids (TDS) were 78.71±4.23%, 81.67±4.76%, 63.20±3.84%, and 9.95±0.81%, respectively. An additional DHS unit was used as a post-treatment process to improve the effluent quality, giving final concentrations of 38.42±2.21 mg/L for COD, 6.12±0.42 mg/L for TSS, 2.79±0.14 mg/L for NH3, and 411.31±25.70 mg/L for TDS. The first DHS unit was responsible for the degradation of COD compounds as an organic carbon source, with NH4+-N as the main N-source, for heterotrophic growth. The second DHS was responsible for eliminating a large portion of NH3, probably due to the occurrence of autotrophic nitrification, converting NH4+ to NO3. Improving the performance of sequential DHS/DHS units could be attributed to operating the entire system at an F/M ratio of 0.18 kg COD/kg MLVSS/d, SRT of 161 d, and HRT of 2.4 h. The system performance to treat wastewater was successfully predicted by the Monod model, giving kinetic parameters of µmax= 0.65 1/h and Ks= 12.13 g/L (R2 0.943). Because natural ventilation was used to supply the sponge with oxygen, treating sewage by the dual DHS/DHS system would cost 2.23 US$/m3. It’s suggested that an additional unit, such as membranes, could be used to reduce the remaining TDS concentrations.

Keywords: long-term continuous feed; operating condition; sewage; sponge-based technology; treatment cost